Wideband code division multiple access (WCDMA) is a third generation (3G) cellular technology that enables the concurrent transmission of a plurality of distinct digital signals via a common RF channel. WCDMA supports a range of communications services that include voice, high speed data and video communications. One such high speed data communications service, which is based on WCDMA technology, is the high speed downlink packet access (HSDPA) service.
WCDMA is a spread spectrum technology in which each digital signal is coded or “spread” across the RF channel bandwidth using a spreading code. Each of the bits in the coded digital signal is referred to as a “chip.” A given base transceiver station (BTS), which concurrently transmits a plurality of distinct digital signals, may encode each of a plurality of distinct digital signals by utilizing a different spreading code for each distinct digital signal. At a typical BTS, each of these spreading codes is referred to as a Walsh code. The Walsh coded digital signal may in turn be scrambled by utilizing a pseudo-noise (PN) bit sequence to generate chips. An example of a PN bit sequence is a Gold code. Each of a plurality of BTS within an RF coverage area may utilize a distinct PN bit sequence. Consequently, Walsh codes may be utilized to distinguish distinct digital signals concurrently transmitted from a given BTS via a common RF channel while PN bit sequences may be utilized to distinguish digital signals transmitted by distinct BTSs. The utilization of Walsh codes and PN sequences may increase RF frequency spectrum utilization by allowing a larger number of wireless communications to occur concurrently within a given RF frequency spectrum. Accordingly, a greater number of users may utilize mobile communication devices, such as mobile telephones, Smart phones and/or wireless computing devices, to communicate concurrently via wireless communication networks.
A user utilizing a mobile communication device may be engaged in a communication session with a user utilizing a first mobile communication device via a base transceiver station within a wireless communication network. For example, the mobile communication device may transmit a digital signal to the base transceiver station, which the base transceiver station may then transmit to a second mobile communication device. The base transceiver station may encode signals received from the second mobile communication device and transmitted to the mobile communication device by utilizing a Walsh code and a PN sequence. The second mobile communication device may receive signals transmitted concurrently by multiple base transceiver stations in addition to the base transceiver station within a given RF coverage area. The second mobile communication device may process the received signals by utilizing a descrambling code that is based on the PN sequence and a despreading code that is based on the Walsh code. In doing so, the second mobile communication device may detect a highest relative signal energy level for signals received from base transceiver station, which comprise a digital signal corresponding to the first mobile communication device.
However, the second mobile communication device may also detect signal energy from the digital signals, which correspond to signals from mobile communication devices other than the first mobile communication device. The other signal energy levels from each of these other mobile communication devices may be approximated by Gaussian white noise, but the aggregate noise signal energy level among the other mobile communication device may increase in proportion to the number of other mobile communication devices whose signals are received at the second mobile communication device. This aggregate noise signal energy level may be referred to as multiple access interference (MAI). The MAI may result from signals transmitted by the base transceiver station, which originate from signal received at the base transceiver station from mobile communication devices other than the first mobile communication device. The MAI may also result from signals transmitted by the base transceiver stations BTSs other than the base transceiver station. The MAI and other sources of noise signal energy may interfere with the ability of the second mobile communication device to successfully decode signals received from the first mobile communication device.
An additional source of noise signal energy may result from multipath interference. The digital signal energy corresponding to the second mobile communication device, which is transmitted by the base transceiver station may disperse in a wavefront referred to as a multipath. Each of the components of the multipath may be referred to as a multipath signal. Each of the multipath signals may experience a different signal propagation path from the base transceiver station to the second mobile communication device. Accordingly, different multipath signals may arrive at different time instants at the second mobile communication device. The time duration, which begins at the time instant that the first multipath signal arrives at the second mobile communication device and ends at the time instant that the last multipath signal arrives at the second mobile communication device, is referred to as a delay spread. The second mobile communication device may utilize a rake receiver that allows the second mobile communication device to receive signal energy from a plurality of multipath signals received within a receive window time duration. The receive window time duration may comprise at least a portion of the delay spread time duration. Multipath signals that are not received within the receive window time duration may also contribute to noise signal energy.